As electronic instruments have recently been small and had high densities, multi-layer circuit boards have been required not only in industrial instruments but also in consumer instruments.
Such circuit boards require a method of interconnecting circuit patterns on plural layers through an inner via-hole and a highly-reliable structure. Japanese Patent Laid-Open Publication No.6-268345 discloses a conventional method of manufacturing a multi-layer circuit board having an inner via-hole made of conductive paste. The conventional method of manufacturing a circuit board having four layers will be described.
First, a method of manufacturing a double-sided circuit board used as a substrate of the multi-layer circuit board will be described. FIG. 6A to FIG. 6G are sectional views showing processes of the conventional method of manufacturing the double-sided circuit board.
Pre-preg sheet 101, a substrate, is made of a composite material including a core and thermosetting epoxy resin impregnated in the core. The core is made of non-woven fabric, such as aromatic polyamide fiber, and has a thickness t101 of 150 μm compressed at a compression rate of about 35%. Pre-preg sheet 101 employs a porous material having vacancy for obtaining a compressive property.
FIG. 6A shows pre-preg sheet 101 having both surfaces onto which releasing films 102a and 102b are bonded, respectively. Respective surfaces of releasing films 102a and 102b are coated with Si-based releasing agent. The films are made of film, such as polyethylene terephthalate film. Through-hole 103 is formed in predetermined positions of pre-preg sheet 101 by a laser machining method, as shown in FIG. 6B. Through-hole 103 are filled with conductive paste 104 by a printing method, as shown in FIG. 6C.
Then, releasing films 102a and 102b are peeled off from the surfaces of pre-preg sheet 101, as shown in FIG. 6D. Metal foils 105a and 105b are placed on the surfaces of pre-preg sheet 101, and are heated and pressurized by hot press, as shown in FIG. 6E. Thus, thickness t102 of pre-preg sheet 101 is reduced to about 100 μm, and pre-preg sheet 101 is bonded to metal foils 105a and 105b, as shown in FIG. 6F. Metal foils 105a and 105b are electrically connected via conductive paste 104 filling through-hole 103.
Metal foils 105a and 105b is selectively etched to form circuit patterns 106a and 106b, thus providing a double-sided circuit board shown in FIG. 6G.
FIG. 7A to FIG. 7D are sectional views showing the conventional method of manufacturing a multi-layer circuit board having four layers as will be described below.
First, as shown in FIG. 7A, double-sided circuit board 110 having circuit patterns 106a and 106b is prepared, and pre-preg sheets 101a and 101b having through-holes 103 filled with conductive paste 104 are prepared. Double-sided circuit board 110 is manufactured by the processes shown in FIG. 6A to FIG. 6G, and pre-preg sheets 101a and 101b are manufactured by the processes shown in FIG. 6A to FIG. 6D.
Then, metal foil 105b, pre-preg sheet 101b, double-sided circuit board 110, pre-preg sheet 101a, and metal foil 105a are positioned and stacked in this order on a laminated plate (not shown), as shown in FIG. 7B.
Then, metal foil 105b, pre-preg sheet 101b, double-sided circuit board 110, pre-preg sheet 101a, and metal foil 105a are heated and pressurized by hot press. Thus, as shown in FIG. 7C, pre-preg sheets 101a and 101b are compressed to have thickness t102, and are bonded to double-sided circuit board 110 and metal foils 105a and 105b. Circuit patterns 106a and 106b are electrically connected to metal foils 105a and 105b via conductive paste 104, respectively.
As shown in FIG. 7D, metal foils 105a and 105b are selectively etched to form circuit patterns 106a and 106b, thus providing four-layer circuit board 120.
A multi-layer circuit board having more than four layers, such as a six-layer circuit board, is obtained by repeating the processes shown in FIG. 7A to FIG. 7D using four-layer circuit board 120 obtained by the processes of FIG. 7A to FIG. 7D instead of double-sided circuit board 110.
In the case that the through-hole has a small diameter and is arranged adjacent to another through-hole by a small pitch for providing a fine circuit board, the conventional method of manufacturing the circuit board has the following problem.
The pre-preg sheet made of porous material has a vacancy to be compressed. When the volume ratio of the vacancy to the pre-preg sheet is large, a portion of the conductive paste intends to be put into the vacancy. The resistance of the conductive paste in the hole accordingly increases, and electrical insulation between the conductive pastes in the adjacent through-hole may be hardly obtained. Therefore, a material having small porosity may preferably used, but the material having the small porosity cannot be compressed at a high compressed rate.
FIG. 8A and FIG. 8B are sectional views of the circuit board formed by the conventional method.
FIG. 8A shows pre-preg sheet 101 having a compression rate of 35%. In FIG. 8A, pre-preg sheet 101 is sufficiently compressed before resin impregnated into pre-preg sheet 101 flows in surface direction D101, so that conductive paste 104 does not flow out of through-hole 103 and has a stable resistance.
In FIG. 8B, pre-preg sheet 101 has a high porosity and a small compressed rate, e.g. smaller than 10%. A compression rate of conductive paste 104 decreases during heating and pressurizing, and conductive paste 104 may flow as denoted by flow 115. A contacting force between conductive particles in conductive paste 104 accordingly decreases.
When the resin in pre-preg sheet 101 melts due to the heating and pressurizing and flows in surface direction D101, conductive paste 104 flows out of through-hole 103. The contacting force between the conductive particles in conductive paste 104 accordingly decreases thus increasing the resistance of a portion of conductive paste 104 in through-hole 103. Then, the connection resistance between metal foils 105a and 105b accordingly increases, thus causing a quality of the circuit board to decline.
In order to solve this problem, the metal foil and a pre-preg sheet having a core and a resin impregnated into the core are stacked, and they are then heated for a predetermined time at a first temperature close to a softening temperature of the resin while being pressurized by a predetermined pressure, and then heated for a predetermined time at a second temperature higher than the first temperature and pressurized.
In this method, the processes at the first temperature to the second temperature are executed continuously, so that the rate of a temperature rise during the processes is restricted. Specifically, when the first temperature varies to the second temperature, the rate of the temperature rise at a temperature at which the resin in the pre-preg sheet melts and flows may be reduced due to a delay of heat conduction through an intermediate material, such as a cushioning material or a SUS plate. The rate of the temperature rise may not reach a predetermined rate. In other words, fluidiity of the resin during molding is not sufficiently secured, so that the pre-preg sheet can hardly be molded especially when the viscosity of the melting resin is high.